Many manufacturers of products requiring a temporary holding, mixing or other fluid processing step as part of the manufacturing process now opt to use disposable bags for holding and contacting the fluid under sterile conditions instead of rigid mixing tanks or vessels. Advantageously, such bags are relatively inexpensive to manufacture, easy to transport, store, and sterilize, and can be disposed of once the operation is complete and any product recovered. Moreover, the use of such bags not only both eliminates the need for cleaning and the chance for cross-contamination, but also reduces the overall process cost.
A frequent problem encountered by users of such bags, which are typically made of one or more layers of thin, fluid-impervious plastic film, is the risk of undesirable leakage. In the case of bioprocessing applications, such leaks result in the loss of sterility inside the bag or the release of potentially hazardous biological material. If not detected, such leakage can thus compromise the entire mixing operation and render the end product unsuitable for use. However, early detection of such leakage can advantageously lead to appropriate corrective action being taken, such as before the end product is distributed.
Up to now, leakage in these types of processing bags has been difficult to detect in a reliable fashion. Usually, such leakage is discovered visually in the form of wetness on the outside surface of the bag, spills on the floor and so on. Of course, this method of detection is not reliable, since such wetness may result from the condensation on the bag outer surface, and unidentified spills can arise from other sources. Also, many leaks cannot be detected visually if the bag is kept in the closed non-transparent container.
In the particular case of bioprocessing, most of the biological fluids held, stored or processed in bags comprise liquids, such as water based electrolytes, that are relatively good electrical conductors. The material comprising the bag, on the other hand, is typically polyethylene or other plastic which is very good electrical insulator. The bags containing biological fluids can have just one layer of plastic film or several layers of film. In the latter case, the fluid touching layer of the bag usually has the best leachable and extractable characteristics to minimize the contamination of the processed fluid with molecules diffused out of the plastic to the fluid. This layer is typically low density polyethylene. However, this innermost layer has poor mechanical strength, and thus one or more additional layers are added to enhance the mechanical strength of the bag.
The external layer(s) are not supposed to touch the processed fluid but provide for the mechanical strength of the bag. Even though those layers are also sterile on the inner side, they may leach substantial amount of organic molecules if they contact fluid as a result of the internal leak. Consequently, the leaks that can develop in the bags can be of two types:                1. Internal leaks where the electrically conductive fluid in the interior compartment of the bag through only one or several layers of the film but does not leak outside of the bag; or        2. External leaks, where the conductive fluid leaks to the outside non-sterile environment.Both types of leaks can potentially contaminate the product in the bag and thus are highly undesirable.        
Accordingly, a need exists for an expedient, yet reliable manner for detecting either or both of these types of leaks, thereby allowing for immediate corrective action to be taken.